Network Working Group E. Burger
Request for Comments: 5438 Unaffiliated
Category: Standards Track H. Khartabil
Ericsson Australia
February 2009
Instant Message Disposition Notification (IMDN)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
Instant Messaging (IM) refers to the transfer of messages between
users in real-time. This document provides a mechanism whereby
endpoints can request Instant Message Disposition Notifications
(IMDN), including delivery, processing, and display notifications,
for page-mode instant messages.
The Common Presence and Instant Messaging (CPIM) data format
specified in RFC 3862 is extended with new header fields that enable
endpoints to request IMDNs. A new message format is also defined to
convey IMDNs.
This document also describes how SIP entities behave using this
extension.
Burger & Khartabil Standards Track [Page 1]

RFC 5438 IMDN February 2009
This specification defines three categories of disposition types:
"delivery", "processing", and "display". Specific disposition types
provide more detailed information. For example, the "delivery"
category includes "delivered" to indicate successful delivery and
"failed" to indicate failure in delivery.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
This document refers generically to the sender of a message in the
masculine (he/him/his) and the recipient of the message in the
feminine (she/her/hers). This convention is purely for convenience
and makes no assumption about the gender of a message sender or
recipient.
3. Terminology
o IM: An Instant Message generated using the Message/CPIM format.
o IMDN: An Instant Message Disposition Notification generated using
the Message/CPIM format that carries an IMDN XML document.
o Message: An IM or an IMDN generated using the Message/CPIM format.
o IM Sender: An endpoint (user agent) generating and sending an IM.
Also, the endpoint request IMDNs for an IM. Quite often, the IM
Sender is the IMDN Recipient. However, that is not always the
case, since the IMDN uses the From header in the CPIM message.
That value is often the IM Sender's Address of Record (AOR). This
address may in fact resolve to different user agents.
o IM Recipient: An endpoint (user agent) that receives IMs. The IM
Recipient, as the node that presumably renders the IM to the user,
generates and sends delivery IMDNs to IMs, if requested by the IM
Sender and allowed by the IM Recipient.
o Endpoint: An IM Sender or an IM Recipient.
o Intermediary: An entity in the network, most often an application
server (including URI-List and store-and-forward servers), that
forwards an IM to its final destination. Intermediaries also can
generate and send processing IMDNs to IMs, if requested by the IM
Sender and allowed by policy.
Burger & Khartabil Standards Track [Page 4]

RFC 5438 IMDN February 2009
o Gateway: An intermediary that translates between different IM
systems that use different protocols.
o IMDN payload: An XML document carrying the disposition
notification information. In this specification, it is of MIME
type "message/imdn+xml".
o Disposition type: This specification defines three categories of
disposition types: "delivery", "processing", and "display".
o Transport Protocol Message: A SIP or other protocol message that
contains an IM or IMDN.
4. Overview
The diagram below shows the basic protocol flow. An IM Sender
creates an IM, adds IMDN request information that the IM Sender is
interested in receiving, and then sends the IM. At a certain point
in time, the IM Recipient or an intermediary determines that the user
or application has received, did not receive, displayed, or otherwise
disposed of the IM. The mechanism by which an IM Recipient
determines its user has read an IM is beyond the scope of this
document. At that point, the IM Recipient or intermediary
automatically generates a notification message to the IM Sender.
This notification message is the Instant Message Disposition
Notification (IMDN).
+--------------+ +--------------+
| IM Sender | | IM Recipient |
|IMDN Recipient| | IMDN Sender |
+--------------+ +--------------+
| |
| |
| 1. IM requesting IMDN |
|-------------------------------------->|
| |
| |
| 2. IMDN (disposition) |
|<--------------------------------------|
| |
| |
Basic IMDN Message Flow
Note the recipient of an IMDN, in some instances, may not be the IM
Sender. This is specifically true for page-mode IMs where the
Address of Record (AOR) of the IM Sender, which is present in the IM,
resolves to a different location or user agent than that from which
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RFC 5438 IMDN February 2009
the IM originated. This could happen, for example, if resolving the
AOR results in forking the request to multiple user agents. For
simplicity, the rest of this document assumes that the IM Sender and
the IMDN Recipient are the same and therefore will refer to both as
the IM Sender.
5. Disposition Types
There are three broad categories of disposition states. They are
delivery, processing, and display.
5.1. Delivery
The delivery notification type indicates whether or not the IM has
been delivered to the IM Recipient. The delivery notification type
can have the following states:
o "delivered" to indicate successful delivery.
o "failed" to indicate failure in delivery.
o "forbidden" to indicate denial for the IM Sender to receive the
requested IMDN. The IM Recipient can send the "forbidden" state,
but usually it is an intermediary that sends the message, if one
configures it to do so. For example, it is possible the
administrator has disallowed IMDNs.
o "error" to indicate an error in determining the fate of an IM.
5.2. Processing
The processing notification type indicates that an intermediary has
processed an IM. The processing notification type can have the
following states:
o "processed" to indicate that the intermediary has performed its
task on the IM. This is a general state of the IM.
o "stored" to indicate that the intermediary stored the IM for later
delivery.
o "forbidden" to indicate denial for the IM Sender to receive the
requested IMDN. The "forbidden" state is sent by an intermediary
that is configured to do so. For example, the administrator has
disallowed IMDNs.
o "error" to indicate an error in determining the fate of an IM.
Burger & Khartabil Standards Track [Page 6]

RFC 5438 IMDN February 20095.3. Display
The display notification type indicates whether or not the IM
Recipient rendered the IM to the user. The display notification type
can have the following states:
o "displayed" to indicate that the IM has been rendered to the user.
o "forbidden" to indicate denial, by the IM Recipient, for the IM
Sender to receive the requested IMDN.
o "error" to indicate an error in determining the fate of an IM.
In addition to text, some IMs may contain audio, video, and still
images. Therefore, the state "displayed" includes the start of
rendering the audio or video file to the user.
Since there is no positive acknowledgement from the user, one cannot
determine if the user actually read the IM. Thus, one cannot use the
protocol described here as a service to prove someone actually read
the IM.
6. New CPIM Header Fields
This specification extends the CPIM data format specified in RFC 3862
[RFC3862]. A new namespace is created as well as a number of new
CPIM header fields.
6.1. CPIM Header Field Namespace
Per CPIM [RFC3862], this specification defines a new namespace for
the CPIM extension header fields defined in the following sections.
The namespace is:
urn:ietf:params:imdn
As per CPIM [RFC3862] requirements, the new header fields defined in
the following sections are prepended, in CPIM messages, by a prefix
assigned to the URN through the NS header field of the CPIM message.
The remainder of this specification always assumes an NS header field
like this one:
NS: imdn <urn:ietf:params:imdn>.
Of course, clients are free to use any prefix while servers and
intermediaries must accept any legal namespace prefix specification.
Burger & Khartabil Standards Track [Page 7]

RFC 5438 IMDN February 20096.2. Disposition-Notification
The IM Sender MUST include the Disposition-Notification header field
to indicate the desire to receive IMDNs from the IM Recipient for
that specific IM. Section 10 defines the syntax.
6.3. Message-ID
The IM Sender MUST include the Message-ID header field in the IM for
which he wishes to receive an IMDN. The Message-ID contains a
globally unique message identifier that the IM Sender can use to
correlate received IMDNs. Because the Message-ID is used by the
sender to correlate IMDNs with their respective IMs, the Message-ID
MUST be selected so that:
o There is a minimal chance of any two Message-IDs accidentally
colliding during the time period within which an IMDN might be
received.
o It is prohibitive for an attacker who has seen one or more valid
Message-IDs to generate additional valid Message-IDs.
The first requirement is a correctness requirement to ensure correct
matching by the sender. The second requirement prevents off-path
attackers from forging IMDNs. In order to meet both of these
requirements, it is RECOMMENDED that Message-IDs be generated using a
cryptographically secure, pseudo-random number generator and contain
at least 64 bits of randomness, thus reducing the chance of a
successful guessing attack to n/2^64, where n is the number of
outstanding valid messages.
When the IM Sender receives an IMDN, it can compare its value with
the value of the <message-id> element present in the IMDN payload.
IMDNs also carry this header field. Note that since the IMDN is
itself an IM, the Message-ID of the IMDN will be different than the
Message-ID of the original IM. Section 10 defines the syntax.
6.4. Original-To
An intermediary MAY insert an Original-To header field into the IM.
The value of the Original-To field MUST be the address of the IM
Receiver. The IM Recipient uses this header to indicate the original
IM address in the IMDNs. The IM Recipient does this by populating
the <original-recipient-uri> element in the IMDN. The intermediary
MUST insert this header if the intermediary changes the CPIM To
header field value. The header field MUST NOT appear more than once
in an IM. The intermediary MUST NOT change this header field value
if it is already present. Section 10 defines the syntax.
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RFC 5438 IMDN February 20096.5. IMDN-Record-Route
An intermediary MAY insert an IMDN-Record-Route header field to the
IM. This enables the intermediary to receive and process the IMDN on
its way back to the IM Sender. The value of the IMDN-Record-Route
header field MUST be the address of the intermediary. Multiple IMDN-
Record-Route header fields can appear in an IM. Section 10 defines
the syntax.
6.6. IMDN-Route
The IMDN-Route header field provides routing information by including
one or more addresses to which to route the IMDN. An intermediary
that needs the IMDN to flow back through the same intermediary MUST
add the IMDN-Record-Route header. When the IM Recipient creates the
corresponding IMDN, the IM Recipient copies the IMDN-Record-Route
headers into corresponding IMDN-Route header fields. Section 10
defines the syntax.
7. Endpoint Behaviour7.1. IM Sender7.1.1. Constructing Instant Messages
An IM is constructed using the CPIM message format defined in RFC3862 [RFC3862].
7.1.1.1. Adding a Message-ID Header Field
If the IM Sender requests the reception of IMDNs, the IM Sender MUST
include a Message-ID header field. This header field enables the IM
Sender to match any IMDNs with their corresponding IMs. See
Section 6.3 for Message-ID uniqueness requirements.
7.1.1.2. Adding a DateTime Header Field
Some devices are not able to retain state over long periods. For
example, mobile devices may have memory or battery limits. Such
limits mean these devices may not be able to, or may choose not to,
keep sent messages for the purposes of correlating IMDNs with sent
IMs. To make some use of IMDN in this case, we add a time stamp to
the IM to indicate when the user sent the message. The IMDN returns
this time stamp to enable the user to correlate the IM with the IMDN
at the human level. We use the DateTime CPIM header field for this
purpose. Thus, if the IM Sender would like an IMDN, the IM Sender
MUST include the DateTime CPIM header field.
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RFC 5438 IMDN February 20097.1.1.3. Adding a Disposition-Notification Header Field
The Disposition-Notification conveys the type of disposition
notification requested by the IM Sender. There are three types of
disposition notification: delivery, processing, and display. The
delivery notification is further subdivided into failure and success
delivery notifications. An IM Sender requests failure delivery
notification by including a Disposition-Notification header field
with value "negative-delivery". Similarly, a success notification is
requested by including a Disposition-Notification header field with
value "positive-delivery". The IM Sender can request both types of
delivery notifications for the same IM.
The IM Sender can request a processing notification by including a
Disposition-Notification header field with value "processing".
The IM Sender can also request a display notification. The IM Sender
MUST include a Disposition-Notification header field with the value
"display" to request a display IMDN.
The absence of this header field or the presence of the header field
with an empty value indicates that the IM Sender is not requesting
any IMDNs. Disposition-Notification header field values are comma-
separated. The IM Sender MAY request more than one type of IMDN for
a single IM.
Future extensions may define other disposition notifications not
defined in this document.
Section 10 describes the formal syntax for the Disposition-
Notification header field. The following is an example CPIM body of
an IM where the IM Sender requests positive and negative delivery
notifications, but not display notification or processing
notifications:
From: Alice <im:alice@example.com>
To: Bob <im:bob@example.com>
NS: imdn <urn:ietf:params:imdn>
imdn.Message-ID: 34jk324j
DateTime: 2006-04-04T12:16:49-05:00
imdn.Disposition-Notification: positive-delivery, negative-delivery
Content-type: text/plain
Content-length: 12
Hello World
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RFC 5438 IMDN February 20097.1.2. Matching IMs with IMDNs
An IM Sender matches an IMDN to an IM by matching the Message-ID
header field value in the IM with the <message-id> element value in
the body of the IMDN. If the IM was delivered to multiple
recipients, the IM Sender uses the <recipient-uri> element and the
<original-recipient-uri> element in the XML body of the IMDN it
received to determine if the IM was sent to multiple recipients and
to identify the IM Recipient that sent the IMDN.
An IM Sender can determine an IMDN is a disposition notification by
noting if the Content-Disposition in the IMDN is "notification".
This does mean the IM Sender MUST understand the Content-Disposition
MIME header in CPIM messages.
7.1.3. Keeping State
This specification does not mandate the IM Sender to keep state for a
sent IM.
Once an IM Sender sends an IM containing an IMDN request, it MAY
preserve the IM context (principally the Message-ID), other user-
identifiable information such as the IM subject or content, and the
date and time it was sent. Without preservation of the IM context,
the IM Sender will not be able to correlate the IMDN with the IM it
sent. The IM Sender may find it impossible to preserve IM state if
it has limited resources or does not have non-volatile memory and
then loses power.
There is, however, the concept of a "Sent Items" box in an
application that stores sent IMs. This "Sent Items" box has the
necessary information and may have a fancy user interface indicating
the state of a sent IM. A unique Message-ID for this purpose proves
to be useful. The length of time for items to remain in the "Sent
Items" box is a user choice. The user is usually free to keep or
delete items from the "Sent Items" box as she pleases or as the
memory on the device reaches capacity.
Clearly, if an IM Sender loses its sent items state (for example, the
user deletes items from the "Sent Items" box), the client may use a
different display strategy in response to apparently unsolicited
IMDNs.
This specification also does not mandate an IM Sender to run any
timers waiting for an IMDN. There are no time limits regarding when
IMDNs may be received.
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RFC 5438 IMDN February 2009
IMDNs may legitimately never be received, so the time between the
sending of an IM and the generation and ultimate receipt of the IMDN
may simply take a very long time. Some clients may choose to purge
the state associated with the sent IM. This is the reason for adding
the time stamp in the IM and having it returned in the IMDN. This
gives the user some opportunity to remember what IM was sent. For
example, if the IMDN indicates that the IM the user sent at 2 p.m.
last Thursday was delivered, the user has a chance to remember that
they sent an IM at 2 p.m. last Thursday.
7.1.4. Aggregation of IMDNs
An IM Sender may send an IM to multiple recipients in one Transport
Protocol Message (typically using a URI-List server [RFC5365]) and
request IMDNs. An IM Sender that requested IMDNs MUST be prepared to
receive multiple aggregated or non-aggregated IMDNs. See Section 8.3
for details.
7.2. IM Recipient7.2.1. Constructing IMDNs
IM Recipients examine the contents of the Disposition-Notification
header field of the CPIM message to determine if the recipient needs
to generate an IMDN for that IM. Disposition-Notification header
fields of CPIM messages can include one or more values. IM
Recipients may need to generate zero, one, or more IMDNs for that IM,
for example, a delivery notification as well as a display
notification. In this case, the IM Recipient MUST be able to
construct multiple IMDNs per IM. An IM Recipient MUST NOT construct
more than one IMDN per disposition type. That is, it must not
generate a delivery notification indicating "delivered" followed by a
delivery notification indicating "failed" for the same IM. If the IM
Sender requested only failure notifications and the IM was
successfully delivered, then no IMDNs will be generated. If the IM
Recipient does not understand a value of the Disposition-Notification
header field, the IM Recipient ignores that value.
The IM Recipient MUST NOT generate "processing" notifications.
A Disposition-Notification header field MUST NOT appear in an IMDN
since it is forbidden to request an IMDN for an IMDN. An IM Sender
MUST ignore a delivery notification request in an IMDN if present.
The IM Sender MUST NOT send an IMDN for an IMDN.
Burger & Khartabil Standards Track [Page 12]

RFC 5438 IMDN February 2009
An IMDN MUST contain a Message-ID header field. The same rules of
uniqueness for the Message-ID header field that appears in an IM
apply to an IMDN. The Message-ID header field in the IMDN is
different and unrelated to the one in the IM.
An IM may contain an IMDN-Record-Route header field (see Section 8
for details). If IMDN-Record-Route header fields appear in the IM,
the IM Recipient constructing the IMDN MUST copy the contents of the
IMDN-Record-Route header fields into IMDN-Route header fields in the
IMDN and maintain their order. The IMDN is then sent to the URI in
the top IMDN-Route header field. IMDN-Record-Route header fields do
not make sense in an IMDN and therefore MUST NOT be placed in an
IMDN. IMDN Recipients MUST ignore it if present.
If there is no IMDN-Record-Route header field, the IM Recipient MUST
send the IMDN to the URI in the From header field.
As stated in CPIM [RFC3862], CPIM messages may need to support MIME
headers other than Content-type. IM Recipients MUST insert a
Content-Disposition header field set to the value "notification".
This indicates to the IM Sender that the message is an IMDN to an IM
it has earlier sent.
7.2.1.1. Constructing Delivery Notifications
The IM Recipient constructs a delivery notification in a similar
fashion as an IM, using a CPIM body [RFC3862] that carries a
Disposition Notification XML document formatted according to the
rules specified in Section 11. The MIME type of the Disposition
Notification XML document is "message/imdn+xml".
Section 10 defines the schema for an IMDN.
The following is an example CPIM body of an IMDN:
From: Bob <im:bob@example.com>
To: Alice <im:alice@example.com>
NS: imdn <urn:ietf:params:imdn>
imdn.Message-ID: d834jied93rf
Content-type: message/imdn+xml
Content-Disposition: notification
Content-length: ...
<?xml version="1.0" encoding="UTF-8"?>
<imdn xmlns="urn:ietf:params:xml:ns:imdn">
<message-id>34jk324j</message-id>
<datetime>2008-04-04T12:16:49-05:00</datetime>
<recipient-uri>im:bob@example.com</recipient-uri>
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RFC 5438 IMDN February 2009
<original-recipient-uri
>im:bob@example.com</original-recipient-uri>
<delivery-notification>
<status>
<delivered/>
</status>
</delivery-notification>
</imdn>
7.2.1.2. Constructing Display Notifications
The IM Recipient constructs a display notification in a similar
fashion as the delivery notification. See Section 7.2.1.1 for
details.
Section 10 defines the schema for an IMDN.
The following is an example:
From: Bob <im:bob@example.com>
To: Alice <im:alice@example.com>
NS: imdn <urn:ietf:params:imdn>
imdn.Message-ID: dfjkleriou432333
Content-type: message/imdn+xml
Content-Disposition: notification
Content-length: ...
<?xml version="1.0" encoding="UTF-8"?>
<imdn xmlns="urn:ietf:params:xml:ns:imdn">
<message-id>34jk324j</message-id>
<datetime>2008-04-04T12:16:49-05:00</datetime>
<recipient-uri>im:bob@example.com</recipient-uri>
<original-recipient-uri
>im:bob@example.com</original-recipient-uri>
<display-notification>
<status>
<displayed/>
</status>
</display-notification>
</imdn>
There are situations where the IM Recipient cannot determine if or
when the IM has been displayed. The IM Recipient in this case
generates a display notification with a <status> value of "error" to
indicate an internal error by the server. Note that the IM Recipient
may choose to ignore any IMDN requests and not send any IMDNs. An IM
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RFC 5438 IMDN February 2009
Recipient may not wish to let a sender know whether or not a
particular message has been displayed to her. This could be a per-
message, per-sender, or programmed policy choice.
8. Intermediary Behaviour
In this context, intermediaries are application servers (including
URI-List and store-and-forward servers) and gateways. A gateway is a
server that translates between different IM systems that use
different protocols.
A URI-List server may change the IM Recipient address from its own to
the address of the final recipient of that IM for every copy it makes
that it sends to the list members (see [RFC5365] for details). In
this case, if the IM Sender is requesting an IMDN, the intermediary
SHOULD add an Original-To header field to the IM, populating it with
the address that was in the CPIM To header field before it was
changed. That is, the intermediary populates the Original-To header
field with the intermediary address. Of course, one may configure an
intermediary to restrict it from rewriting or populating the
Original-To field. An intermediary MUST NOT add an Original-To
header field if one already exists. An intermediary MAY have an
administrative configuration to not reveal the original Request-URI,
and as such, MUST NOT add an Original-To header.
An IM reply for a page-mode IM is not linked in any way to the
initial IM and can end up at a different user agent from where the
initial IM originated, depending on how the recipient URI gets
resolved. Therefore, IM replies may traverse different
intermediaries. An IMDN, on the other hand, needs to traverse the
same intermediaries as the IM itself since those intermediaries may
be required to report negative delivery notifications if the IM was
not delivered successfully. Some of those intermediaries are, for
example, store-and-forward servers that may report that an IM has
been processed and later report that the IM has failed to be
delivered.
For the reasons stated above, an intermediary MAY choose to remain on
the path of IMDNs for a specific IM. It can do so by adding a CPIM
IMDN-Record-Route header field as the top IMDN-Record-Route header
field. The value of this field MUST be the intermediary's own
address. An intermediary that does not support this extension will
obviously not add the IMDN-Record-Route header field. This allows
IMDNs to traverse directly from the IM Recipient to the IM Sender
even if the IM traversed an intermediary not supporting this
extension.
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RFC 5438 IMDN February 2009
An intermediary receiving an IMDN checks the top IMDN-Route header
field. If that header field carries the intermediary address, the
intermediary removes that value and forwards the IMDN to the address
indicated in the new top IMDN-Route header field. If no additional
IMDN-Route header fields are present, the IMDN is forwarded to the
address in the CPIM To header field.
An intermediary MUST remove any information about the final
recipients of a list if the list membership is not disclosed. The
intermediary does that by removing the <recipient-uri> element and/or
<original-recipient-uri> element from the body of the IMDN before
forwarding it to the IM Sender.
8.1. Constructing Processing Notifications
Intermediaries are the only entities that construct processing
notifications. They do so only if the IM Sender has requested a
"processing" notification by including a Disposition-Notification
header field with value "processing".
The intermediary can create and send "processing" notifications
indicating that an IM has been processed or stored. The intermediary
MUST NOT send more than one IMDN for the same disposition type --
i.e., it must not send a "processing" notification indicating that an
IM is being "processed" followed by another IMDN indicating that the
same IM is "stored".
An intermediary constructs a "processing" notification in a similar
fashion as the IM Recipient constructs a delivery notification. See
Section 7.2.1.1 for details.
The following is an example:
Content-type: Message/CPIM
From: Bob <im:bob@example.com>
To: Alice <im:alice@example.com>
Content-type: message/imdn+xml
Content-Disposition: notification
Content-length: ...
<?xml version="1.0" encoding="UTF-8"?>
<imdn xmlns="urn:ietf:params:xml:ns:imdn">
<message-id>34jk324j</message-id>
<datetime>2008-04-04T12:16:49-05:00</datetime>
<recipient-uri>im:bob@example.com</recipient-uri>
<original-recipient-uri
>im:bob@example.com</original-recipient-uri>
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RFC 5438 IMDN February 2009
<processing-notification>
<status>
<processed/>
</status>
</processing-notification>
</imdn>
There are situations where the intermediary cannot know the fate of
an IM. The intermediary in this case generates a processing
notification with a <status> value of "error" to indicate so.
8.2. Constructing Delivery Notifications
Intermediaries MAY construct negative delivery notifications. They
do so only if the IM Sender has requested a "negative-delivery"
notification by including a Disposition-Notification header field
with value "negative-delivery" AND an error was returned for that IM.
The intermediary can create and send "negative-delivery"
notifications indicating that an IM has failed to be delivered. The
intermediary MUST NOT send more than one IMDN for the same
disposition type -- i.e., it must not send a "failed" notification
indicating that an IM has failed followed by another IMDN indicating
that an IMDN is "forbidden".
An intermediary constructs a "negative-delivery" notification much
like the IM Recipient. See Section 7.2.1.1 for details.
8.3. Aggregation of IMDNs
As previously described, URI-List servers are intermediaries.
A URI-List server may choose (using local policy) to aggregate IMDNs
or it may send individual IMDNs instead. When a URI-List server
receives an IM and decides to aggregate IMDNs, it can wait for a
configurable period of time or until all recipients have sent the
IMDN, whichever comes first, before it sends an aggregated IMDN.
Note that some IMDNs, for example "displayed" notifications, may
never come due to user settings. How long to wait before sending an
aggregated IMDN and before a URI-List server removes state for that
IM is an administrator configuration and implementation issue.
A URI-List server MAY choose to send multiple aggregated IMDNs. A
timer can be started, and when it fires, the URI-List server can
aggregate whatever IMDNs it has so far for that IM, send the
aggregated IMDN, and restart the timer for the next batch. This is
needed for scenarios where the IM Sender has requested more than one
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RFC 5438 IMDN February 2009
IMDN for a specific IM -- for example, delivery notifications as well
as display notifications -- or when the URI-List server is short on
resources and chooses to prioritise forwarding IMs over IMDNs.
A second timer can be running, and when it fires, the state of the IM
is deleted. In this case, the URI-List server consumes any IMDNs
that might arrive after that time.
Please note the references to timers in the above paragraphs are not
normative and are only present to help describe one way one might
implement aggregation.
A URI-List server MAY aggregate IMDNs for the case where the list
membership information is not disclosed. There may be scenarios
where the URI-List server starts sending aggregated IMDNs and
switches to individual ones or visa versa. A timer firing often may
in fact have that effect.
The aggregated IMDN is constructed using the multipart/mixed MIME
type and including as individual payloads all the IMDNS that were
received as message/imdn+xml.
Below is an example of aggregated IMDNs.
From: Bob <im:bob@example.com>
To: Alice <im:alice@example.com>
NS: imdn <urn:ietf:params:imdn>
imdn.Message-ID: d834jied93rf
Content-type: multipart/mixed;
boundary="imdn-boundary"
Content-Disposition: notification
Content-length: ...
--imdn-boundary
Content-type: message/imdn+xml
<?xml version="1.0" encoding="UTF-8"?>
<imdn xmlns="urn:ietf:params:xml:ns:imdn">
<message-id>34jk324j</message-id>
<datetime>2008-04-04T12:16:49-05:00</datetime>
<recipient-uri>im:bob@example.com</recipient-uri>
<original-recipient-uri
>im:bob@example.com</original-recipient-uri>
<delivery-notification>
<status>
<delivered/>
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RFC 5438 IMDN February 2009
</status>
</delivery-notification>
</imdn>
--imdn-boundary
Content-type: message/imdn+xml
<?xml version="1.0" encoding="UTF-8"?>
<imdn xmlns="urn:ietf:params:xml:ns:imdn">
<message-id>34jk324j</message-id>
<datetime>2008-04-04T12:16:49-05:00</datetime>
<recipient-uri>im:bob@example.com</recipient-uri>
<original-recipient-uri
>im:bob@example.com</original-recipient-uri>
<display-notification>
<status>
<displayed/>
</status>
</display-notification>
</imdn>
--imdn-boundary
9. Identifying Messages
Messages are typically carried in a transport protocol like SIP
[RFC3261]. If the payload carried by the transport protocol does not
contain any parts of type Message/CPIM, then the message is an IM.
If the payload contains any parts of type Message/CPIM, and none of
those parts contains a payload that is of type "message/imdn+xml",
the message is an IM. It is not valid to attempt to carry both an IM
and an IMDN in a multipart payload in a single transport protocol
message.
A message is identified as a delivery notification by examining its
contents. The message is a delivery notification if the Content-type
header field present has a value of "message/imdn+xml", the Content-
Disposition header field has a value of "notification", and the
<delivery-notification> element appears in that XML body.
A message is identified as a processing notification or display
notification in a similar fashion as a delivery notification. The
difference is that, for a processing notification, the <processing-
notification> element appears in the XML body. For a display
notification, the <display-notification> element appears in the XML
body.
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RFC 5438 IMDN February 200910. Header Fields Formal Syntax
The following syntax specification uses the message header field
syntax as described in Section 3 of RFC 3862 [RFC3862].
Header field syntax is described without a namespace qualification.
Following the rules in RFC 3862 [RFC3862], header field names and
other text are case sensitive and MUST be used as given, using
exactly the indicated upper-case and lower-case letters.
Disposition-Notification =
"Disposition-Notification" ": "
[(notify-req *(COMMA notify-req))]
notify-req =
("negative-delivery" / "positive-delivery" /
"processing" / "display" / Token) *(SEMI disp-notify-params)
disp-notify-params = Ext-param
Message-ID = "Message-ID" ": " Token
Original-To = "Original-To" ": " [ Formal-name ] "<" URI ">"
IMDN-Record-Route =
"IMDN-Record-Route" ": " [ Formal-name ] "<" URI ">"
IMDN-Route = "IMDN-Route" ": " [ Formal-name ] "<" URI ">"
SEMI = *SP ";" *SP ; semicolon
COMMA = *SP "," *SP ; comma
11. IMDN Format11.1. Structure of an XML-Encoded IMDN Payload
An IMDN payload is an XML document [XML] that MUST be well-formed and
MUST be valid according to schemas, including extension schemas,
available to the validater and applicable to the XML document. The
IMDN payload MUST be based on XML 1.0 and MUST be encoded using
UTF-8.
The schema allows qualified extension elements in several positions
other than the "urn:ietf:params:xml:ns:imdn" namespace. To maintain
forwards compatibility (i.e., newer instance documents can be used by
existing consumers), the new specifications MUST NOT extend the
allowable content of this specification. The backwards compatibility
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RFC 5438 IMDN February 2009
(i.e., existing instance documents can also be used by updated, new
consumers) MAY break if there are conflicts with the existing
qualified names of extension elements and possible future
specifications. The IETF MAY specify new extension elements within
the "sub-namespace" of "urn:ietf:params:xml:ns:" for this message/
imdn+xml MIME type.
Possible future specifications can add new element definitions with
the combine="interleave" pattern. When multiple elements of this new
type are then allowed, the new definition MUST contain the
<zeroOrMore> cardinality rule. If the new specification does allow
only a single new element, the <optional> cardinality rule MUST be
used. These cardinality requirements maintain the backwards
compatibility of existing instance documents with newer consumers.
Also, the new specification MUST then redefine either the "anyIMDN"
extension or the individual extension points that reference it, so
that new element definitions do not match with this redefined and
more limited wildcard pattern.
The namespace identifier for elements defined by this specification
is a URN [URN], using the namespace identifier 'ietf' defined by
[URN_NS] and extended by [IANA]. This urn is:
urn:ietf:params:xml:ns:imdn.
This namespace declaration indicates the namespace on which the IMDN
is based.
The root element is <imdn>. The <imdn> element has sub-elements,
namely <message-id>, <datetime>, <recipient-uri>, <original-
recipient-uri>, <subject>, and one of <delivery-notification>,
<processing-notification>, or <display-notification>. A <status>
also appears as a sub-element of <delivery-notification>,
<processing-notification>, and <display-notification>. The elements
are described in detail in the following sections.
<imdn> can be extended in the future to include new disposition
notification types or other elements, as described in Section 11.1.9.
11.1.1. The <message-id> Element
The <message-id> element is mandatory according to the XML schema and
carries the message ID that appeared in the Message-ID header field
of the IM.
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RFC 5438 IMDN February 200911.1.2. The <datetime> Element
The <datetime> element is mandatory and carries the date and time the
IM was sent (not the IMDN). This information is obtained from the
DateTime header field of the IM.
11.1.3. The <recipient-uri> Element
The <recipient-uri> element is optional and carries the URI of the
final recipient. This information is obtained from the CPIM To
header field of the IM.
11.1.4. The <original-recipient-uri> Element
The <original-recipient-uri> element is optional and carries the URI
of the original recipient. It MUST be present if the IM carried the
Original-To header field. This information is obtained from the
Original-To header field of the IM.
11.1.5. The <subject> Element
The <subject> element is optional. If present, it MUST carry the
text and language attributes that were in the Subject header field,
if any. This allows for a human-level correlation between an IM and
an IMDN. If there are more than one Subject header fields in an IM,
selecting any one of them to place in the IMDN payload <subject>
element will suffice. The sender then needs to compare Subject
header fields until a match or not match is determined.
11.1.6. The <delivery-notification>, <processing-notification>, and
<display-notification> Elements
The appearance of one of the <delivery-notification>, <processing-
notification>, and <display-notification> elements is mandatory and
carries the disposition type that the IM Sender requested and is
being reported. It carries the sub-element <status>.
11.1.7. The <status> Element
The <status> element is mandatory and carries the result of the
disposition request. For notification type <delivery-notification>,
it can carry one of the sub-elements <delivered>, <failed>,
<forbidden>, or <error>. For notification type <display-
notification>, it can carry one of the sub-elements <displayed>,
<forbidden>, or <error>. For notification type <processing-
notification>, it can carry one of the sub-elements <processed>,
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RFC 5438 IMDN February 2009
</element>
</define>
</grammar>
12. Transporting Messages Using SIP12.1. Endpoint Behaviour12.1.1. Sending Requests
The IM Sender constructs a SIP MESSAGE request using RFC 3428
[RFC3428]. The Content-type header field indicates the MIME type of
the request payload. When using this extension, the Content-type
header field MUST be of MIME type "message/cpim" [RFC3862] for both
IMs and IMDNs. The IM Sender constructs the payload according to
Section 7.
The IM Sender constructs a SIP MESSAGE request to multiple recipients
in a similar manner as a SIP MESSAGE request to a single recipient.
"Multiple-Recipient MESSAGE Requests in SIP" [RFC5365] describes the
differences.
IM Senders can remain anonymous. For example, the sender can set the
SIP From header field of the SIP message to an anonymous URI. As
there is no return address, anonymous IM Senders SHOULD NOT request
disposition notifications. An IM Recipient MAY ignore such a request
if the IM Sender is anonymous.
12.1.2. Sending Responses
An endpoint receiving a SIP MESSAGE request constructs a SIP response
according to RFC 3428 [RFC3428]. Of course, an endpoint will send a
SIP response to the MESSAGE request regardless of the type of message
(IM or IMDN) it has received or the disposition type for which it has
been asked.
12.1.3. Receiving Requests12.1.3.1. Instant Message
A SIP MESSAGE request is identified as an IM by examining its
contents according to Section 9.
If an IM Recipient received a SIP MESSAGE request that is an IM
requesting a positive-delivery notification, and that IM Recipient
has constructed and sent a SIP 2xx class response, it MAY generate a
positive-delivery notification after making sure that the IM has been
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RFC 5438 IMDN February 2009
delivered to the user or application. A gateway, for example, can
generate a 2xx response before the final recipient received the IM.
The IM Recipient constructs a positive-delivery notification
according to Section 7.2.1.1. The IM Recipient places the message as
the payload in a SIP MESSAGE request.
If an IM Recipient received a SIP MESSAGE request that is an IM
requesting a negative-delivery, and that IM Recipient has constructed
and sent a 2xx class response, it SHOULD generate a negative-delivery
notification if it learnt that the final recipient or application did
not receive the IM (a gateway, for example, can generate a 2xx
response before it has an error response from downstream or before
any internal timers fire waiting for a response). The negative-
delivery notification is constructed according to Section 7.2.1.1.
The message is then placed as the payload in a SIP MESSAGE request.
If an IM Recipient received a SIP MESSAGE request that is an IM
requesting a negative-delivery notification, and the IM Recipient has
constructed and sent a non-2xx final response, it MUST NOT generate a
negative-delivery notification.
If an IM Recipient received a SIP MESSAGE request that is an IM
requesting a display notification, and that IM Recipient has
constructed and sent a SIP 2xx class response, it MAY generate a
display notification after making sure that the IM has been presented
to the user or application. It is outside the scope of this document
to discuss how a determination can be made whether the IM has been
read. Note that the decision whether or not to send a display
notification can be left to the user. An application may allow a
user to configure such a choice. The IM Recipient constructs the
display notification according to Section 7.2.1.2. The IM Recipient
places the message as the payload in a SIP MESSAGE request.
For IMDNs, the IM Recipient populates the SIP Request-URI and the SIP
To header field using the address that appeared in the SIP From
header field in the IM.
12.1.3.2. Delivery Notification
A SIP MESSAGE request is identified as a delivery notification by
examining its contents according to Section 9.
12.1.3.3. Display Notification
A SIP MESSAGE request is identified as a display notification by
examining its contents according to Section 9.
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RFC 5438 IMDN February 200912.2. Intermediary Behaviour
In this context, intermediaries include application servers
(including URI-List and store-and-forward servers) and gateways. SIP
Proxies MUST NOT generate IMDNs but MUST forward them like any other
SIP request.
Intermediaries forward a SIP MESSAGE request to multiple recipients
according to [RFC5365].
If an intermediary receives an IM, the intermediary examines the
body. If the body is of type "message/cpim", the intermediary then
looks for a Disposition-Notification CPIM header field in the
message. If the Disposition-Notification CPIM header field has
either the value "positive-delivery" or "negative-delivery", and, in
processing the IM, the intermediary generates a SIP 2xx class
response to the MESSAGE request, then the intermediary performs the
following actions.
If the Disposition-Notification header field contains a value of
"positive-delivery", the intermediary MUST NOT generate a delivery
notification if it receives a SIP 2xx class response for the sent IM.
Just because a downstream entity received a MESSAGE request does not
mean the message was relayed to its ultimate destination or was
delivered. Thus, the intermediary cannot say delivery occurred just
because it received a 2xx response.
If the Disposition-Notification header field contains a value of
"negative-delivery", the intermediary SHOULD generate a delivery
notification if it receives a SIP 4xx, 5xx, or 6xx class final
response for the sent IM. If it has received a SIP 2xx class
response followed by a negative-delivery notification, the
intermediary forwards that negative-delivery notification or
aggregates it.
If the Disposition-Notification header field contains a value of
"processing", the intermediary MAY generate a processing notification
after it has forwarded or stored the IM. The rest of the procedures
in Section 8.1 apply.
The procedure for generating such an IMDN is the same as that of an
IM Recipient (Section 7.2.1.1).
The <recipient-uri> element of the XML body is populated with the URI
of the IM Recipient.
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RFC 5438 IMDN February 2009
If an intermediary receives a SIP MESSAGE request carrying a positive
delivery notification or a display notification, it forwards it using
the rules in Section 8.
13. Transporting Messages using MSRP
The Message Session Relay Protocol (MSRP) [RFC4975] already provides
a built-in mechanism to supply positive and negative delivery
reports. These reports do not provide built-in display or processing
notifications. However, these notifications in session-mode are not
as useful as they are for page-mode. This is because the base use
case for MSRP is that the recipient user agent immediately renders
SEND requests sequentially, providing the session experience. This
is unlike page-mode requests where a user has to actively initiate
the display of the message. That is, they need to click on a button,
open a message, and so on to read the message.
If new requirements arise in the future determining the need for IMDN
in MSRP, new specifications can be drafted.
14. Security Considerations
IMDNs provide a fine-grained view of the activity of the IM
Recipient, and thus deserve particularly careful confidentiality
protection so that only the intended recipient of the IMDN will
receive the IMDN. In most cases, the intended recipient of the IMDN
is the IM Sender.
Since the IM transport protocol carries the IMDN, all security
considerations of the underlying IM protocol also apply to the IMDNs.
The threats in the IMDN system, over and beyond the threats inherent
to IM, include the following:
o A malicious endpoint attempts to send messages to a user that
would normally not wish to receive messages from that endpoint by
convincing the IMDN system to "bounce" an IMDN from an
unsuspecting endpoint to the user.
o A malicious endpoint attempts to flood an IM Sender with IMDNs by
convincing a URI-List server to send IMDNs to an unsuspecting IM
Sender.
o A malicious intermediary or node attempts to flood a target node
with IMDNs by inserting the target's address in the From field or
IMDN-Record-Route field.
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RFC 5438 IMDN February 2009
o A malicious node in the network attempts to modify an IMDN from an
IM Recipient.
o A malicious intermediary attempts to forward an IMDN from an IM
Recipient to the IM Sender, where the IM Recipient would not
normally forward the IMDN to that IM Sender if the IM Recipient
knew the identity of the IM Sender.
o A malicious endpoint attempts to discover the Request-URI of an
endpoint beyond an intermediary, where the endpoint would normally
wish to keep its identity private from the malicious endpoint.
o A malicious node in the network attempts to eavesdrop on IMDN
traffic to, for example, learn Request-URI or traffic pattern
information.
o A malicious node in the network attempts to stage a denial-of-
service attack on an intermediary by requesting a large list
expansion.
The protocol cannot protect against attacks that include the
following:
o A malicious intermediary directly revealing the identity of a
downstream endpoint that would not normally wish its identity
revealed. Keeping such information private is an intermediary
implementation issue.
o A malicious IM Recipient alters the time of the IMDN. There is no
protocol mechanism for ensuring that the IM Recipient does not lie
about the time or purposely holds an IMDN for transmission to make
it appear that the IM displayed to the user was read later than it
actually was.
o A deletion attack on an IMDN. This is a trade-off between privacy
and security. The privacy considerations allow the IM Recipient
to silently ignore an IMDN request. Any mechanism that would
reliably indicate that a malicious node deleted an IM Recipient's
IMDN would also serve the purpose of detecting an IM Recipient
that chose not to issue an IMDN.
To combat eavesdropping, modification, and man-in-the-middle attacks,
we require some level of authentication and integrity protections.
That said, there are circumstances where strong integrity would be
overkill. The presumption is that the IM Sender has, and sets the
expectation for, the level of protection. The procedures for
integrity protection are as follows.
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RFC 5438 IMDN February 2009
o If the IM Recipient has a certificate, it MUST sign the IMDN.
Signing the IMDN provides integrity protection. While an
intermediary can replace the IMDN body, the IM Sender (the
recipient of the IMDN) can validate the signature and note the
IMDN does not come directly from the IM Receiver. This is not a
problem if the IM Sender trusts the intermediary. Likewise, an
IMDN in response to a signed IM without a signature indicates
something bad might have happened.
o If the IM is encrypted, the IM Recipient or intermediary MUST
encrypt the IMDN body, as an attacker may attempt to discern the
user's activity profile and identity from sniffing IMDNs on the
network.
o The two above rules are cumulative.
The IM Recipient or intermediary MUST be capable of accessing the IM
Sender's public certificate in order to verify the signature in the
IM.
CPIM security considerations [RFC3862] apply here, as this is an
extension of CPIM. In order to make the IMDN mechanism independent
of the transport protocol, the Working Group made the design choice
of putting routing information into the IMDN application-layer
payload. One consequence of this choice is it eliminates the
possibility of having end-to-end encryption.
An attacker can mount a distributed denial-of-service attack on a
node by sending lots of IMs to the node with IMDN requests. Note
that this is the same problem as there is without IMDN; IMDN simply
linearly increases the load on the node under attack. One can
mitigate, but not eliminate, this threat by the endpoint immediately
ignoring requests that are not authenticated.
One way to address the potential for a malicious node to use the IMDN
system to anonymize attacks is to log all IMDN requests on the IM
Recipient user agent. This allows for tracking of attacks, if only
after they occur. Note this also puts a burden on the IM Recipient
user agent host. Limited user agents may not be able to preserve
much of a log.
Likewise, an attacker can mount a denial-of-service attack on an
intermediary by asking the intermediary to explode a large list.
The following security considerations apply when using IMDNs.
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RFC 5438 IMDN February 200914.1. Forgery
IMs can be forged. To protect against that, an IM can be signed. An
intermediary that receives a signed message and needs to modify any
part of it that is included in the signature (like adding an
Original-To header field to the CPIM header fields) MUST consume the
IM and create a new copy of it that the intermediary signs itself.
IMDNs may be forged as easily as ordinary IMs. Endpoints and
intermediaries that wish to make automatic use of IMDNs should take
appropriate precautions to minimize the potential damage from denial-
of-service attacks. Security threats related to forged IMDNs include
the sending of a falsified IMDN when the indicated disposition of the
IM has not actually occurred. For example, display notification
could be forged to indicate that an IM has been displayed to the
Recipient. Unsolicited IMDNs is also another form of forgery.
14.2. Confidentiality
There may be cases where an IM Recipient does not wish to reveal that
she has received, or in fact read, the IM. In this situation, it is
acceptable for the IM Recipient to silently ignore requests for an
IMDN. It is strongly RECOMMENDED that the IM Recipient obtain the
user's consent before sending an IMDN. Circumstances where the IM
Recipient does not ask for the user's consent include IM systems
that, for regulatory reasons, are required to issue an IMDN, such as
in the health care field or financial community.
An IM Recipient can obtain such consent by a prompt or dialog box on
a per-IM basis, globally through the user's setting of a preference,
or another, user-configurable mechanism. The user might also
indicate globally that IMDNs are never to be sent or that a
"forbidden" IMDN status is always sent in response to a request for
an IMDN.
There are situations where a user sends an IM and requests IMDNs to a
list whose member information is not disclosed. In this situation,
the user will learn of the list members. Therefore, in this case,
the URI-List server MUST remove any information about list members.
If the number of members in the list is also not disclosed, the URI-
List server MUST only deliver one aggregated IMDN. Alternatively,
the URI-list server MAY reject the IM.
It is possible for a list server to not understand IMDN. IM
Recipients may note the To header field is a list name and not the IM
Recipient's name. In this case, the IM Recipient can take the
appropriate action if it wishes to keep its identity private.
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RFC 5438 IMDN February 2009
An unencrypted IMDN could reveal confidential information about an
encrypted IM. The same level of security applied to an IM MUST be
applied to its IMDNs. For example, if an IM is signed and encrypted,
the IMDN must be signed and encrypted.
14.3. IMDN as a Certified Delivery Service
IMDNs cannot be relied on as a guarantee that an IM was or was not
seen by the user. Even if IMDNs are not actively forged, they may be
lost in transit. Moreover, the IM Recipient may bypass the IMDN
issuing mechanism through policy or manipulation of their user agent
Server.
15. IANA Considerations15.1. message/imdn+xml MIME TYPE
This document registers a new MIME type "message/imdn+xml", and
registers a new XML namespace.
This specification follows the guidelines of RFC 3023 [RFC3023].
MIME media type: message
MIME subtype name: imdn+xml
Mandatory parameters: none
Optional parameters: Same as charset parameter application/xml as
specified in RFC 3023 [RFC3023].
Encoding considerations: Same as encoding considerations of
application/xml as specified in RFC 3023 [RFC3023].
Security considerations: See Section 10 of RFC 3023 [RFC3023] and
Section 14 of this document.
Interoperability considerations: none
Published specification: This document
Applications which use this media type: This media type is used to
support CPIM-based instant Messaging.
Additional information: none
Magic number: none
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RFC 5438 IMDN February 2009
File extension: .cl or .xml
Macintosh file type code: "TEXT"
Personal and email address for further information: Hisham Khartabil
(hisham.khartabil@gmail.com)
Intended Usage: COMMON
Author/change controller: The IETF
15.2. XML Registration
This section registers a new XML namespace and schema, as per
guidelines in the IETF XML Registry [IANA].
URI: urn:ietf:params:xml:ns:imdn
XML: The schema for this namespace is in Section 11.1.9 above.
Registrant Contact: IETF, SIMPLE working group, Hisham Khartabil
(hisham.khartabil@gmail.com)
15.3. URN Registration for IMDN Header Parameters
Per [RFC3553], please establish the following registry. New entries
to the registry are Specification Required.
Registry name: urn:ietf:params:imdn
Specification: RFC 5438. Additional values may be defined by a
Standards Action [RFC5226] that updates or obsoletes RFC 5438.
Repository: RFC 5438
Index value: Values subordinate to urn:ietf:params:imdn require RFC
publication. The index value is the IMDN header name. The index
value must follow the rules for a legal IMDN header name. In
particular, the IMDN header name, and thus the index value to
register, must be a string of octets taken from the restricted set of
US-ASCII characters per Section 3.1 of [RFC3553]. The index value is
case sensitive.
URN Formation: The URI for a header is formed from its name by
a) replacing any non-URN characters (as defined by RFC 2141 [URN])
with the corresponding '%hh' escape sequence (per RFC 3986
[RFC3986]) and
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